Household project calculator
How many gallons of paint do you really need?
You measure a room, stand in front of the paint display, and wonder whether to buy two cans or four. Guess too low and you’re back at the store. Guess too high and you’ve got a shelf of leftovers.
Let’s account for the room, doors, windows, coats, and a sensible extra allowance.
Tell me about the room
These start with an ordinary rectangular room. If the layout is complicated, add up the wall sections separately rather than forcing them into one rectangle.
How the estimate works
The wall area of a rectangular room is 2 × (length + width) × wall height. Then subtract the area of openings you won’t paint, multiply by the number of coats, allow for a little waste, and divide by the coverage listed on the paint can.
For quick planning, this page allows 20 square feet per door and 15 per window, like an example published by Benjamin Moore. Actual openings vary. If yours are unusually large, measure each opening and calculate the net surface area directly.
Sherwin-Williams says a gallon of paint typically covers around 350–400 square feet for one coat. The calculator starts at 350 but lets you replace that number with the figure for the product you are considering. Coverage changes with surface texture, condition, application method and paint.
A couple of things a calculator can’t tell you
A dark-to-light color change may need extra coats or a primer. Rough plaster, brick and very porous walls can use more paint than a smooth previously painted wall. A fresh wall may call for different preparation than one that’s been painted many times.
Ceiling paint is commonly a different sheen or formulation from wall paint. The calculator rounds those requirements separately when you include the ceiling. Buying a gallon in two different colors is not the same as buying two gallons of one color.
Real-world check: Look at the can’s coverage and prep instructions, and match your room’s surface conditions before shopping. Protect nearby surfaces and follow ventilation and product safety instructions while painting.
Questions people ask
How much paint for a 12-by-10 room?
With 8-foot walls, the gross wall area is 352 square feet. One door and two windows at the guide allowances subtract 50 square feet, leaving 302. Two coats and 10% extra at 350 square feet per gallon comes to about 1.90 gallons, so two whole gallons for the walls.
Should I subtract windows and doors?
Yes, for a closer estimate. But subtract the measured opening sizes for unusual windows, double doors or large sliding doors. This calculator uses typical allowances for quick planning.
Does this estimate include trim and primer?
No. Measure baseboards, molding, doors or primer separately. Their sheen, color, coverage and can size may differ.
Sources: Sherwin-Williams paint calculator FAQ and Benjamin Moore paint estimate guide.
Supplies worth checking before you start
Paint is only part of the trip to the store. A good roller cover, a decent angled brush, painter’s tape, drop cloths, and a tray can save you a second trip. Choose supplies for your wall surface and the paint manufacturer’s directions.
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As an Amazon Associate I earn from qualifying purchases. This is an optional search link, not a recommendation of a particular untested kit. Check compatibility and current Amazon prices yourself. The paint calculator is free.